I too have been looking for one of these, over the past year or so. They sure do seem very hard to find. Where I live in Oregon, many of the originals were purchased as surplus and used as game carriers on hunting trips. Over the years most were beaten to pieces hauling mule deer over the lava beds, or else drastically modified and thus ruined.
Since I am not really a collector so much as a living historian, I need one to demonstrate how they work. So until I find an original I am making one. The springs were standard wagon seat springs. The tires and wheels were standard 26' bicycle items. All of this is still available. The rest of the carrier is a little harder to come by. Right now I am pinning down the details of those parts and I am sure they can be made up. I have another query on this site requesting information, so any details anyone has I would greatly appreciate. I will submit some photos when it is done.